Morphology and magnetic properties of island-like Co and Ni films obtained by de-wetting
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- Tiberto, P., Gupta, S., Bianco, S. et al. J Nanopart Res (2011) 13: 245. doi:10.1007/s11051-010-0023-2
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The morphological, structural, and magnetic properties of Co and Ni films of different thicknesses grown by RF sputtering on a Si–SiO substrate and submitted to controlled diffusion of atoms on the substrate (de-wetting) are studied through X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and alternating-gradient magnetometry. For both metals, de-wetting treatment leads to the growth of non-percolating, metallic nanoislands characterized by a distribution of sizes and aspect ratios. XRD spectra reveal a polycrystalline multi-component structure evolving by effect of de-wetting and directly affecting the magnetic properties of films. The magnetic response after de-wetting is consistent with the formation of a nanogranular magnetic phase characterized by a complex, thickness-dependent magnetic behavior originating from the simultaneous presence of superparamagnetic and blocked-particle contributions. At intermediate film thickness (around 10 nm), a notable enhancement in magnetic coercivity is observed for both metals with respect to the values measured in precursor films and in their bulk counterparts.
KeywordsMagnetic nanoparticlesMagnetic thin filmsDe-wetting techniqueCoercive field
De-wetting of thin metallic films deposited on Si–SiO substrates by controlled diffusion of metallic atoms (Oh et al. 2009) has been widely studied from the standpoint of fundamental science and technological applications (Bouville et al. 2007; Gadkari et al. 2005). The combined effects of annealing at constant temperature and differences in surface tension may cause a continuous metallic film to split into an array of non-percolating nanoislands, whose size is in proportion to the original film thickness (Oh et al. 2009). Despite the unquestionable interest in the investigation of the physical and chemical factors which affect the de-wetting of thin metal films (Sieradzki et al. 2001), this effect has been proven detrimental in nanoelectronic device technology, causing failures due to overheating (Srolovitz and Safran 1986). However, de-wetting can be exploited as a self-organization process for nanostructuring.
Magnetic thin films and nanoparticles have become a widely investigated class of materials owing to their potential use in nanoelectronics and data storage (Nalwa 2002). Various techniques have been exploited to obtain nanostructured magnetic thin films, such as for instance conventional top-down lithography methods (Smyth et al. 1988; Martin et al. 2003; Saavedra et al. 2010). However, these techniques are costly, time consuming, and do not allow pattern wide-area patterning.
Bottom-up fabrication methods such as self-assembling may represent a valid alternative to conventional lithographic process. In this way, quite ordered arrangements of magnetic nanostructures may be obtained, whose size and distance can in principle be easily controlled (Goncharov et al. 2005). In this context, the de-wetting process may be exploited as a particular self-assembly technique suitable to create magnetic nanostructures out of precursor thin films grown on Si substrates covered by with a thermally grown SiO2 layer. Arrays of magnetic-metal nanoparticles on insulating substrates obtained by de-wetting are particularly interesting as prospective catalysts in the growth of carbon nanotubes embedding or decorated by magnetic nanoparticles, aimed to technological applications such as orientable structural materials and functional materials (Chiolerio et al. 2008; Soldano et al. 2008).
This study is conducted to investigate ferromagnetic thin films of 3D transition metals submitted to de-wetting to promote the formation of nanoislands. In particular, our study is focused on the morphological, structural and magnetic properties of island-like ferromagnetic Co and Ni films.
A number of contributions aimed to explore the interplay between microstructure and magnetic properties of Co and Ni films grown on planar substrates have been published. Ultrathin Co and Ni films (up to 10 nm) were investigated by surface magneto-optic methods (Chang et al 2007; Shern et al. 2004); thicker Co and Ni films (up to 700 nanometers) were studied in detail through a variety of techniques (Munford et al. 2002; Kumar et al. 2009). This paper, however, is focussed on the evolution from continuous to island-like films by effect of de-wetting.
Tailoring the formation of fcc and/or hcp Co and Ni nanoparticles (i.e., controlling their sizes and distances) may offer great opportunity for studying and exploiting their magnetic behavior (typically ranging from superparamagnetic to magnetically blocked depending on particle size, composition, and temperature).
Experimental: materials and methods
Thin films of Co having thickness ranging from 3.5 to 30 nm were deposited by RF magnetron sputtering at a pressure of 10 mTorr of Ar (base pressure ~10−7 Torr) and microwave power of 75 W at ambient temperature. Likewise, thin films of Ni with varying thickness ranging 1–20 nm were grown using RF magnetron sputtering technique in similar conditions. All films were grown on commercial Si whose surface was previously coated by thermally grown SiO2 of thickness ~120 nm. Prior to metal thin films deposition, Si wafers were ultrasonically cleaned in organic solvents and rinsed in de-ionized water. Clean wafers were then immersed in HF to remove native SiO2. The cleaned wafers were blown dry using N2 gas and used for thermally grow SiO2 and subsequently sputter metal films. Thin film thickness was evaluated by atomic force microscopy (AFM-DME instruments) in non-contact mode on the edge of a step, obtained after the lift-off of a polymeric mask. Each thickness evaluation was done executing a multi-line mean, thus reducing the estimate error to around 5%.
As-deposited films were annealed at constant temperature of 850 °C in 1 mTorr vacuum level to generate island-like structures whose thickness was measured using standard bearing-height routines available from the AFM software.
The films surface morphology, texture, grain size, and domains were analyzed using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analytical techniques prior to and post de-wetting.
X-ray diffraction technique was used to determine the crystalline structure of the films (Panalytical PW1140–PW3020, Cu Kα X-ray source). The scans were performed in a parallel beam configuration (grazing angle 0.5°), in order to minimize the substrate contribution to the observed diffracted intensities. XPS characterization was performed using a VersaProbe5600 (monochromatic source, Al anode 1486.6 eV).
Magnetic hysteresis loop measurements at room temperature for homogeneous (as-deposited) and island-like (after de-wetting) films were analyzed by using an alternating-gradient field magnetometer (AGFM) in the field range −20 kOe < H < +20 kOe. The diamagnetic contribution of the sample holder and substrates were carefully subtracted from the measured curves. The field was applied in both parallel and perpendicular to the film plane and the magnetization always being measured along a direction parallel to the magnetic field.
Results and discussion
In the case of Ni, two lognormal curves centered at 41 and 245 nm (black lines) are enough to fit the histogram. In this case, the average nanoparticle size and distance are d = 127 and a = 275 nm.
Morphological data of some selected Co and Ni films prior to and after de-wetting
Continuous films: nominal thicknessa (nm)
De-wetted films: mean equivalent diameter db (nm)
De-wetted films: mean island thickness tc (nm)
X-ray diffraction and X-ray photoelectron spectroscopy
The control of crystalline orientation and microstructure is desirable since this directly influences the magnetic properties and corresponding properties such as anisotropy, coercivity, and device functionality (Teranishi et al. 1997; Hirose et al. 1997; Ohsawa et al. 1999).
It is worth noting that in both Co and Ni de-wetted films, the slight increase in nanoparticle size with increasing film thickness observed in thicker films from X-ray diffractograms (see “X-ray diffraction and X-ray photoelectron spectroscopy” section) has quite negligible effects on the hysteresis properties. The largest magnetic changes occur in thin de-wetted films (nominal thickness below 8 m) where no significant conclusion can be safely drawn from X-ray diffractograms.
In summary, the de-wetting of homogeneous metallic Co and Ni films is effective in producing island-like films exhibiting various magnetic states that can be tailored for a number of subsequent applications, including the growth of CNT arrays.
The morphological and magnetic properties Co and Ni films with varying thickness have been investigated prior to and post de-wetting. The nanoparticle size dispersion evidenced by SEM image analysis brings about a complex magnetic behavior which originates from the superposition of superparamagnetic and blocked-particle contributions, and depends on Co and Ni film thickness. In de-wetted films, the maximum coercivity appears at intermediate thickness. For both metals, XRD spectra revealed polycrystalline multi-component structure directly affecting the magnetic properties.
One of the authors (S.G.) is thankful for a research fellowship from Politecnico di Torino during her visit.